I have evaluated various IAg plating products (full qualification), and I used the method of placing the PWBs inside a container of moistened modeling clay, as I learned from a paper by Dave Hillman, in fact. What I learned is that it does not take a lot of Sulfur exposure from a quantity standpoint to begin to show a reaction in the finish, but the evaluation made it very clear that the finish was NOT affected by any short-term exposure to any of several different airborne sources, including soaps, clay, etc. It usually took at least a week of constant exposure to some pretty high levels before you could see a visible reaction.
I also used direct contact with various tapes, rubber gloves (not nitrile though), paper, different types of plastic bags and "saran wrap", and others. What we learned:
1. Direct contact with certain tapes, papers, and Humidity Indicator Cards and certain types of desiccant bags (but not all of them) can cause heavy corrosion reaction, sometimes enough to impact solderability. It only took two weeks of direct exposure inside of a wrapped brick to reduce solderability.
2. I could not find a plastic bag or wrap that caused any visible reaction
3. The moist clay exposure inside of a closed plastic container took about 10 days before you could see a reaction.
4. The rubber gloves had no effect from normal handling, in fact, for some strange reason the areas heavily handled around the edges of the PWB actually showed a minor improvement in solder wetting using the wetting tests within IPC-TM-650 (6mm paste deposit, reflowed).
5. If the rubber gloves were left in contact with the PWB for a week, it definitely showed a reaction (turned slightly yellow), but had NO effect on solder wetting using the same wetting spread test directly on the affected areas.
6. Just like exposing your automobile to a Minnesota winter, certain cars will rot faster than others, and certain IAg products (and plating processes) were much more resistant than others.
7. We qualified two IAg products for use on programs. Both have worked very, very well, but only if the appropriate handling/storage, and processing methods are used.
8. Even if slightly tarnished such that some areas are slightly yellow, they all soldered up just fine.
We strictly enforce the use of nitrile gloves, the handling only by the fixtures whenever possible, and only by the edges otherwise. We have not seen any wetting issues if this is done.
I know that there are things I have seen and heard of such as leaving uncoated IAg CCAs inside of humidity chambers filled with condensation that can cause issues. Common sense prevails.
-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of David Hillman
Sent: Tuesday, January 14, 2020 9:55 AM
To: [log in to unmask]
Subject: Re: [TN] [External] Re: [TN] Need advice : sulphur level on Gloves/ESD mat and reference standards
Hi team - I'll just add a couple of comments to the conversation. The tarnish typically found on immersion silver board finishes is the result of boards being wet/damp from processing which leads to a brown/black visual tarnish due to reactions with the air. The majority of the time the tarnish is cosmetic and does not impact the solderability of the board. If it is not cosmetic then you will find the solderability is a problem pretty easily. The gloves we use are not sulfur free - they are Micro Grip nitrile gloves from VWR International (part number 40101-350) and we haven't seen an issue with them causing tarnishing. Of course, everyone should be handling a board by its edges as much as possible too. I would look at your board storage and handling practices as possible root cause sources for the tarnishing. Good luck.
Dave Hillman
Collins Aerospace
On Tue, Jan 14, 2020 at 7:42 AM Douglas Pauls < [log in to unmask]> wrote:
> "Last Century"? Thanks for turning me from old to ancient Bev.
>
> As Bev indicated, there are no standards in our industry for this kind
> of thing. The recommended ionic levels from Foresite, PAL, or other
> test labs are for circuit assemblies or bare circuit boards, not for
> amounts in gloves. We use standard latex or nitrile gloves here and a
> lot of what we use is immersion silver. We have not seen the need for special gloves.
>
> In terms of functional impact, if it is just tarnish, then there is no
> functional impact. Silver oxide conducts as well as pure silver. All
> you have is a cosmetic issue when people collectively lose their minds
> looking at tarnished silver.
>
> I suspect what happened is somebody somewhere thought "sulfur is bad
> for silver" (which it is), "so we need sulfur free gloves" (which you don't).
>
> *Douglas Pauls *| Principal Materials and Process Engr | Advanced
> Operations Engineering
>
> *COLLINS AEROSPACE*
>
> 400 Collins Road NE, MS 108-101, Cedar Rapids, IA 52498 USA
>
> *Tel:* +1 319 295 2109 | *Mobile: *+1 319 431 3773
>
> [log in to unmask]
>
> [log in to unmask] for all Export Compliant Items
>
>
> On Tue, Jan 14, 2020 at 7:31 AM Bev Christian
> <[log in to unmask]
> >
> wrote:
>
> > First, do you like to be call GA or Tan?
> > a) & b) There are no standards. Shoot, we don't have any "official"
> > standards for sodium, potassium, fluoride, chloride, bromide,
> > nitrate, nitrate, phosphate or sulfate; unless they are in IEC
> > standards that I don't know about. We have the suggestions of Doug
> > and Terry from the last century and those from Delco and a couple of
> > other sets.
> > Certainly the measurement of Doug Pauls, etc. are professionally
> > carried out.
> > c) Only cosmetic
> >
> > I have never heard anyone ask for sulfur free gloves. I am not aware
> > of what is available. I would start with asking a VWR or Fisher rep
> > (large
> chemical
> > suppliers with extensive catalogues) .
> >
> > Regards,
> > Bev
> >
> >
> > -----Original Message-----
> > From: TechNet <[log in to unmask]> On Behalf Of Tan Geok Ang
> > Sent: January 14, 2020 6:34 AM
> > To: [log in to unmask]
> > Subject: [TN] Need advice : sulphur level on Gloves/ESD mat and
> > reference standards
> >
> > Will really appreciate advice on the following;
> >
> > a) how much of sulphur/calcium, etc. on gloves/ESD Mat is acceptable
> > and what level is considered harmful in handling of PCBA/module with
> > sliver plated surface
> >
> >
> > b) whether there are any "standards" or references for "compliance"
> > check for sulphur, and is there such measurements professionally
> > carried out
> >
> >
> > c) What will be the impacts on the PCBA when silver plating become
> tarnish?
> >
> >
> > There are sulphur free gloves but it is not easily
> > accessible/available
> in
> > Singapore and it is expensive.
> >
> >
> > Thanks.
> >
> >
> > GA Tan
> >
>
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